Where to place a PIR / Motion sensor in a lighting circuit?

Discussion in 'Electricians' Talk' started by Nick Birch, Apr 15, 2018.

  1. Nick Birch

    Nick Birch New Member

    I have bought a PIR (9695V) for our upstairs landing lights.

    The circuit is as follows:

    Three switches: a switch downstairs, another switch upstairs as well as a dimmer switch upstairs.
    Two led lights upstairs on the landing.

    Does the PIR need to be wired in between the last switch and the first light or can it go between the two lights in the circuit?

    I am not sure which of the two upstairs switches is the last switch and without lifting a lot of boards in the loft I'm not sure how I can tell.

    I have fitted a PIR before but this was in the garage so installation was a lot simpler.

    Any advice would be greatly appreciated.
  2. robertpstubbs

    robertpstubbs Active Member

    How you wire the PIR in would depend on what its purpose is, which isn't obvious to me from your post.

    With regard to the switch order, this could be determined by loosening off the switches and seeing what wires are connected. A middle switch would be an intermediate one. The end switches would be 2-way and one would supply the lights.
  3. Nick Birch

    Nick Birch New Member

    Thanks for the reply,

    The PIR is for the landing lights, which at the moment are on permanently so the kids can find their way to the bathroom in the middle of the night. So the PIR would switch on the lights when needed.

    So would the PIR need to go between the end switch supplying the lights?
  4. The PIR will almost certainly require a neutral supply too, and good chance this doesn't exist behind your light switch. Ergo, you are stuffed.

    You also clearly haven't the beginnings of an idea how to wire this, so I fear you are on dodgy safety ground.

    I mean, where would this PIR be mounted? How would you run wires to it?

    Do you have a power socket on the landing? If so, that would be your best bet - either a permanent LED soft-glow light, or else good chance you can get a plug-in motion-activated unit.

    If you don't have a socket, then use a battery-powered unit - it'll last ages. Better still, a PV-charged one, and mount it close to a window.
  5. robertpstubbs

    robertpstubbs Active Member

    I know of blocks of flats where there are push-to-make switches on the staircases which turn the lights on for a set period of time. I assume PIRs could be used instead of push-to-makes. But I don't know how a combination of on/off switches and PIRs would work?
  6. Nick Birch

    Nick Birch New Member

    Robert, after your first reply I checked behind the face plates and it is clear which is the last switch before the light. I can see the twin and earth going up - which tallies with the instructions supplied with the PIR.

    The PIR I fitted in the garage is a similar setup but with fewer switches and lights, that's worked for over 15 years with no problem so cant see there being an issue.

    I appreciate all your help, thanks again.
  7. joinerjohn1

    joinerjohn1 Screwfix Select

    There will be however a neutral (or bunch of them) at the actual light itself. Where would you want a PIR ? Not gonna want it mounted beside the light switch are we? More than likely it would be mounted in the ceiling , so not really a problem for the OP? Plenty of neutrals to pick up DA
  8. spinlondon

    spinlondon Well-Known Member

    Some PIRs replace the switch.
  9. Bazza

    Bazza Well-Known Member

    But not in a two or three way switch arrangement, not without a relay to do it.
    One could just wire the PIR on as an over-ride to the existing switch arrangement. Not perfect as the light switches wont have any effect if the PIR is activated, so there's potential for the light to be left on for long periods.

    @Nick Birch Just buy the kids a head torch each.
  10. robertpstubbs

    robertpstubbs Active Member

    Using terms like "last switch" is possibly ambiguous.

    In some 3-switch setups the juice comes down from the light fitting to one 2-way switch, then through the intermediate switch to the last? 2-way switch, then back through the intermediate switch box to the first 2-way switch and back up to power the light.

    In other 3-switch setups the juice comes straight to a 2-way switch, goes through the intermediate switch to the last 2-way switch, and then up to power the light.
  11. Bazza

    Bazza Well-Known Member

    I’ve installed one where the feed to light was at the intermediate switch. That was in the middle of the switch run.
  12. robertpstubbs

    robertpstubbs Active Member

    Was that using 3+E?

    3-switch wiring.jpg
  13. Bazza

    Bazza Well-Known Member


    3c&e between the switches. 2c&e from the light to the switch (live and switched live).
    Standard loop in at the lamp wiring.
    Not sure what your drawing is for?
  14. HarDeBloodyHarHar

    HarDeBloodyHarHar Active Member

    Surely the pir should be between the rose and the light bulb?
  15. Bazza

    Bazza Well-Known Member

  16. spinlondon

    spinlondon Well-Known Member

    Do they make PIR that you fix between the rose and the lamp?
    Do you have to replace the 2 core flex with 3 core?
    One of the problems with this thread, is the lack of information.
    What type of PIR is intended to be installed?
    Will the PIR be replacing a switch?
    Will the PIR be controlled by a switch, or will it work independently?
  17. retiredsparks

    retiredsparks Well-Known Member

    Potty under bed...never did me any harm....lol
  18. PhilSo

    PhilSo Active Member

    Buy the children a Star Wars torch each.

    PhilSo ( Illuminated )
  19. Sparkielev

    Sparkielev Well-Known Member

    Buy a light bulb with a sensor built into it

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